Deterministic lateral displacement (DLD) devices have great potential for the separation and sorting of various suspended particles based on their size, shape, deformability, and other intrinsic properties. Currently, the basic idea for the separation mechanism is that the structure and geometry of DLDs uniquely determine the flow field, which in turn defines a critical particle size and the particle lateral displacement within a device. We employ numerical simulations using coarse-grained mesoscopic methods and two-dimensional models to elucidate the dynamics of both rigid spherical particles and deformable red blood cells (RBCs) in different DLD geometries. Several shapes of pillars, including circular, diamond, square, and triangular structures, and a few particle sizes are considered. The simulation results show that a critical particle size can be well defined for rigid spherical particles and depends on the details of the DLD structure and the corresponding flow field within the device. However, non-isotropic and deformable particles such as RBCs exhibit much more complex dynamics within a DLD device, which cannot properly be described by a single parameter such as the critical size. The dynamics and deformation of soft particles within a DLD device become also important, indicating that not only size sorting, but additional sorting targets (e.g., shape, deformability, internal viscosity) are possible.

Recent advances in cell sorting aim at the development of novel methods that are sensitive to various mechanical properties of cells. Microfluidic technologies have a great potential for cell sorting; however, the design of many micro-devices is based on theories developed for rigid spherical particles with size as a separation parameter. Clearly, most bioparticles are non-spherical and deformable and therefore exhibit a much more intricate behavior in fluid flow than rigid spheres. Here, we demonstrate the use of cells’ mechanical and dynamical properties as biomarkers for separation by employing a combination of mesoscale hydrodynamic simulations and microfluidic experiments. The dynamic behavior of red blood cells (RBCs) within deterministic lateral displacement (DLD) devices is investigated for different device geometries and viscosity contrasts between the intra-cellular fluid and suspending medium. We find that the viscosity contrast and associated cell dynamics clearly determine the RBC trajectory through a DLD device. Simulation results compare well to experiments and provide new insights into the physical mechanisms which govern the sorting of non-spherical and deformable cells in DLD devices. Finally, we discuss the implications of cell dynamics for sorting schemes based on properties other than cell size, such as mechanics and morphology.

We present a systematic dissipative particle dynamics (DPD) study on the phase behavior, structure, and dynamics of rodlike mesogens. In addition to a rigid fused-bead-chain model with RATTLE constraint method, we also construct a semirigid model in which the flexibility is controlled by the bending constant of k(φ). Using this notation, the rigid model has an infinite bending constant of k(φ)=∞. Within the parameter space studied, both two kinds of models exhibit the nematic and smectic-A phases in addition to the isotropic and solid phases. All of the phase transitions are accompanied by the discontinuities in the thermodynamical, structural, and dynamical quantities and the hysteresis around the transition points, and are therefore first order. Note that the obtained solid state exhibits an in-layer tetragonal packing due to the high density. For the rigid model, the simulations show that the liquid crystal phases can be observed for mesogens with at least five beads and the nematic phase is the first one to appear. More importantly, the phase diagram of seven-bead-chain models is obtained as a function of k(φ) and temperature. It is found that decreasing the value of k(φ) reduces the anisotropy of molecular shape and the orientational ordering, and thereby shifts the liquid crystal phases to the lower temperature end of the phase diagram. Due to the different k(φ) dependence of phase transition temperatures, the nematic phase range exhibits a more marked narrowing than the smectic-A phase as k(φ) is reduced, implying that the flexibility has a destabilizing effect on the nematic and smectic-A phases. We also have investigated the anisotropic translational diffusion in liquid crystal phases and its temperature and flexibility dependence. In our study, we find that the phases formed, their statical and dynamic properties, as well as the transition properties are in close accord with those observations in real thermotropic liquid crystals. It is clear that both the rigid and semirigid models we used are valuable models with which to study the behavior of thermotropic liquid crystals using DPD algorithm.

Dissipative particle dynamics simulations have been conducted to study the anchoring transitions of nematic liquid crystals in the presence of a rod-coil amphiphilic monolayer at the aqueous-liquid crystal interface. Instead of amphiphile interfacial coverage, the repulsion interaction parameter (a MR ) between the mesogens and rod blocks of the amphiphiles is used as a tunable and quantitative parameter to control the anchoring transition. Depending on a complicated interplay between the interfacial interactions and the packing effects, we have observed a novel anchoring transition sequence of planar-tilted-homeotropic-tilted-planar when continuously decreasing the value of a MR .

Sorting cells based on their intrinsic properties is a highly desirable objective, since changes in cell deformability are often associated with various stress conditions and diseases. Deterministic lateral displacement (DLD) devices offer high precision for rigid spherical particles, while their success in sorting deformable particles remains limited due to the complexity of cell traversal in DLDs. We employ mesoscopic hydrodynamics simulations and demonstrate prominent advantages of sharpedged DLD obstacles for probing deformability properties of red blood cells (RBCs). By consecutive sharpening of the pillar shape from circular to diamond to triangular geometry, a pronounced cell bending around an edge is achieved, serving as a deformability sensor. Bending around the edge is the primary mechanism, which governs the traversal of RBCs through such DLD device. This strategy requires an appropriate degree of cell bending by fluid stresses, which can be controlled by the flow rate, and exhibits good sensitivity to moderate changes in cell deformability. We expect that similar mechanisms should be applicable for the development of novel DLD devices that target intrinsic properties of many other cells.

An important step in diagnostics is the isolation of specific cells and microorganisms of interest from blood. Since such bioparticles are often present at very low concentrations, throughput needs to be as high as possible. In addition, to ensure simplicity, a minimum of sample preparation is important. Therefore, sorting schemes that function for whole blood are highly desirable. Deterministic lateral displacement (DLD) devices have proven to be very precise and versatile in terms of a wide range of sorting parameters. To better understand how DLD devices perform for blood as the hematocrit increases, we carry out measurements and simulations for spherical particles in the micrometer range which move through DLD arrays for different flow velocities and hematocrits ranging from pure buffer to concentrated erythrocyte suspensions mimicking whole blood. We find that the separation function of the DLD array is sustained even though the blood cells introduce a shift in the trajectories and a significant dispersion for particles whose diameters are close to the critical size in the device. Simulations qualitatively replicate our experimental observations and help us identify fundamental mechanisms for the effect of hematocrit on the performance of the DLD device.

Deterministic lateral displacement (DLD) microfluidic devices promise versatile and precise processing of biological samples. However, this prospect has been realized so far only for rigid spherical particles and remains limited for biological cells due to the complexity of cell dynamics and deformation in microfluidic flow. We employ mesoscopic hydrodynamics simulations of red blood cells (RBC) in DLD devices with circular posts to better understand the interplay between cell behavior in complex microfluidic flow and sorting capabilities of such devices. We construct a mode diagram of RBC behavior (e.g. displacement, zig-zagging, and intermediate modes) and identify several regimes of RBC dynamics (e.g. tumbling, tank-treading, and trilobe motion). Furthermore, we link the complex interaction dynamics of RBCs with the post to their effective cell size and discuss relevant physical mechanisms governing the dynamic cell states. In conclusion, sorting of RBCs in DLD devices based on their shear elasticity is in general possible, but requires fine-tuning of flow conditions to targeted mechanical properties of the RBCs.

Dissipative particle dynamics simulations have been performed to study the coassembly behavior of short carbon nanotubes and diblock copolymers in concentrated solutions by modeling nanotubes as cylindrical nanoparticles (CNPs). With varying the hydrophobic/hydrophilic ratio of the diblock copolymers (A m B n ) at fixed concentration and fixed chain length (m + n = 15), the influence of the addition of CNPs on the lyotropic mesophases including micellar, hexagonal, and lamellar phases has been examined in detail, in comparison with the reference systems involving spherical nanoparticles (SNPs). The simulations reveal that the one-dimensional nature of CNPs plays an important role in determining the final coassembly superstructures. It is interesting to find that the addition of CNPs has significantly increased the stability of the hexagonal phase of coassemblies by inducing the phase transformations of the micellar-to-hexagonal in A 3 B 12 solutions and the lamellar-to-hexagonal in A 7 B 8 solutions, whereas the later morphology change is missing in the presence of SNPs. In addition, both CNPs and SNPs could be selectively accommodated into the hydrophobic layers of the lamellar phases but exhibit different spatial and orientational arrangements. The present study may provide valuable guidance for the design and fabrication of patterned polymer nanocomposites with welldispersed and aligned one-dimensional nanoparticles.